1. Field of the Invention
This invention relates to bilateral transmission systems and, more particularly, to adaptive frequency-sensitive active hybrids for such systems.
2. Description of the Prior Art
Hybrid circuits have been used in communications networks where it is necessary to couple a bidirectional 2-wire line to unidirectional sections of a 4-wire line. In a telephone system, for example, bidirectional signals are carried over the 2-wire subscriber loop but must be split into separate transmit and receive unidirectional signals at the central office and at the subscriber terminal. Various forms of hybrid circuits have been developed to achieve this function.
Standard hybrids for analog voice frequency signals utilize multiwinding transformers arranged to couple signals from the transmission port to the 2-wire access port and to couple signals from the 2-wire access port to the receiving port. By balancing the windings, however, signals imposed on the transmission port are canceled at the receiving port. Such hybrid transformers are comparatively large in size, are high in cost and have frequency-dependent transmission characteristics.
To overcome these disadvantages, various forms of transformerless or electronic hybrids have been proposed. In addition to providing the transmission ports described above, such a hybrid must also present to the 2-wire access port an impedance equal to the characteristic impedance of the connected 2-wire line. To a first approximation, this impedance has heretofore been assumed to be a constant, wholly resistive impedance and simple resistors have been utilized to simulate this characteristic impedance. It is clear, however, that the characteristic impedance of any 2-wire line has certain frequency-sensitive variations which, if not taken into account, produce distortions and imbalances in the transmitted signals. These distortions and imbalances can often be ignored when the hybrid circuit is utilized for the transmission of narrowband analog voice signals such as are used in standard telephony. The frequency-sensitive characteristic impedance tends to vary with the gauge of the wires in the telephone loop.
In modern telephone systems, it has become necessary to connect wideband digital data terminals to such telephone loops. The hybrid transformer and resistive electric hybrids have proved unsuitable for the adequate transmission of such broadband digital signals. One such electric hybrid is disclosed in J. F. Regan U.S. Pat. No. 4,064,377, granted Dec. 20, 1977.